1. Field of the Invention
The present invention relates to a laser apparatus having a structure of an external cavity type semiconductor laser, in particular, to the laser apparatus which is capable of detecting the variation of a wavelength of laser light emitted therefrom.
2. Description of the Related Art
In recent years, since a laser apparatus has many features such as small size and low power consumption, the laser apparatus has been widely used for many information devices. For example, a single mode laser is used for a holographic data storage (HDS). In the HDS, one laser beam is split into two beams by a beam splitter and then the split beams are combined again on a record medium. Using interference of the two beams, data are recorded.
As a light source with which data are hologram recorded and reproduced, a gas laser or a second harmonic generation (SHG) laser is mainly used as a single mode light source. However, when a semiconductor laser such as a laser diode (LD) which generates multi mode laser light is combined with an external resonator, the laser can generate single mode laser light. As a result, this laser can be used as a light source with which data are hologram recorded and reproduced.
Next, with reference to FIG. 1, the structure of a Littrow type laser apparatus which contains a typical external cavity type semiconductor laser will be described. FIG. 1 is a plane view showing the laser apparatus that is designated by reference numeral 200. The structure of the laser apparatus 200 is the same as the structure of a laser apparatus described in L. Ricci, et al., “A compact grating-stabilized diode laser system for atomicphysics”, Optics Communications, 117 1995, pp 541-549.
In the laser apparatus 200, longitudinal multiple mode laser light emitted from a laser diode 201 is collimated by a collimate lens 202. The collimated light enters a reflection type diffraction grating (hereinafter referred to as the grating) 203. The grating 203 outputs first order diffracted light of the incident light. A first order diffracted light having a predetermined wavelength corresponding to an arrangement angle of the grating 203 is inversely injected into the laser diode 201 through the collimate lens 202. As a result, the laser diode 201 resonates with the injected first order diffracted light and emits single mode light (zero-th order light designated by arrow F). The wavelength of the emitted light is the same as the wavelength of the light that returns from the grating 203.
The grating 203 is held by a support portion 204. The support portion 204 has a groove 206. By rotating a screw 205 disposed on the support portion 204, the gap of the groove 206 is partly widened or narrowed. As a result, the horizontal arrangement angle of the grating 203 slightly varies. The reflection angle of the first order light reflected by the grating 203 depends on the wavelength of the laser light emitted from the laser diode 201. By adjusting the angle of the grating 203 for first order light having a desired wavelength that returns to the laser diode 201, laser light having the desired wavelength can be generated.
A similar mechanism is disposed so as to adjust the vertical angle of the grating 203. The support portion 204 which holds the grating 203 is held by a support portion 207. The support portion 207 has a groove (not shown). By rotating a screw 208 disposed on the support portion 207, the gap of the groove is partly widened or narrowed. Thus, the vertical arrangement angle of the grating 203 is slightly varied.
In this case, the laser diode 201 is for example a blue laser diode. In addition, the external cavity type semiconductor laser having the foregoing structure can be used for applications such as a holography memory writer which uses single mode laser light.
Next, with reference to a graph shown in FIG. 2, the relationship between laser power and wavelength of laser light which is output from the external cavity type semiconductor laser apparatus described in FIG. 1 will be described. The horizontal axis of the graph shown in FIG. 2 denotes the laser power of the laser light in mW, whereas the vertical axis of the graph denotes the wavelength of the laser light in nm. As is clear from FIG. 2, as the laser power of the laser light increases, the wavelength thereof varies nearly in a saw tooth wave shape.
The external cavity type semiconductor laser apparatus has an external cavity mode hop region and an inner semiconductor laser chip mode hop region. In the external cavity mode hop region, as the laser power increases, the wavelength of the laser light that is emitted gradually increases. In the inner semiconductor laser chip mode hop region, as the laser power increases, the wavelength of the laser light that is emitted sharply decreases. As the laser power of the laser light increases, the wavelength thereof discretely varies to some extent.
When the laser power is around 30 mW, the external cavity type semiconductor laser apparatus emits laser light having a single wavelength, namely perfectly single mode laser light. However, when the laser power of the laser apparatus is around 32 mW, it emits laser light having three wavelengths, namely three-mode laser light. When the laser power of the laser apparatus is around 35 mW in the inner semiconductor laser chip mode hop region, the laser apparatus emits laser light having six wavelengths, namely six-mode laser light—three modes at a wavelength of around 409.75 nm and three modes at a wavelength of around 409.715 nm.
FIG. 3A, FIG. 3B, FIG. 3C, and FIG. 3D show several spectra of laser light. As was described above, in the external cavity mode hop region of which the wavelength of laser light gradually increases, spectra as shown in FIG. 3A, FIG. 3B, and FIG. 3C occur. On the other hand, in the inner semiconductor laser chip mode hop region where the laser power is around 35 mW, a spectrum as shown in FIG. 3D occurs.
When these types of laser light are used for the HDS, the three-mode laser light (as shown in FIG. 3A) and the two-mode laser light (as shown in FIG. 3B) which are generated with a laser power of 32 mW indicate the same record and reproduction characteristics as the perfect single-mode laser light (a spectrum light in FIG. 3c). Thus, these types of laser light can be used as single-mode laser light. In this case, the perfect signal mode laser light that occurs with a laser power of around 30 mW and the three-mode laser light and the two-mode laser light that occur with a laser power of around 32 mW are together referred to as usable mode laser light.
On the other hand, in the six mode state which occurs with a laser power of around 35 mW as shown in FIG. 3D, since two three-mode regions are spaced apart by around 40 pm, it is difficult to hologram record data. In this case, such six-mode laser light is referred to as unusable mode laser light.
The region in which usable mode laser light is obtained nearly corresponds to the external cavity mode hop region. The region in which unusable mode laser light is obtained nearly corresponds to the inner semiconductor laser chip mode hop region. As is clear from the graph shown in FIG. 2, the region in which usable mode laser light is obtained is much wider than the region in which unusable mode laser light is obtained. Thus, when the unusable mode laser light can be effectively removed, it is quite possible to use the external cavity type semiconductor laser for the HDS.
In addition, characteristics of laser power and wavelength of laser light shown in FIG. 2 depend on the inner temperature of the external cavity type semiconductor laser. When the temperature of the semiconductor laser is not constant, the value of the laser power with which unusable mode laser light occurs varies. Thus, in related art, the inner temperature of the external cavity type semiconductor laser is almost kept constant so that the region in which unusable mode laser light is generated does not vary. In addition, the laser power in the region is not used.
However, according to the method of the related art, to prevent the external cavity type semiconductor laser from emitting the unusable mode laser light, it is necessary to control the laser power thereof while keeping the inner temperature thereof almost constant. Thus, the structure and control of the laser apparatus become complicated.
Although the laser power of the external cavity type semiconductor laser may be controlled with a detected result of the wavelength of laser light, the wavelength detecting device of related art is very large and expensive. Thus, this method is not suitable for applications such as the HDS.
To solve such a problem, the applicant of the present patent application proposed an apparatus and method that has a simple structure and that is capable of detecting the variation of a wavelength for 0.04 nm of laser light emitted from an external cavity type semiconductor laser. Specifically, the applicant proposed to detect the wavelength of laser light emitted from an external cavity type semiconductor laser with an optical wedge.
In the proposed wavelength detecting device, a two-divided detector is disposed on an optical path of light reflected on the front surface and rear surface of the optical wedge. Reflected light of the optical wedge causes interference fringes to occur. The brightness of the interference fringes varies in a sine shape. The phases of the interference fringes move with the wavelength of the laser light. When the interference fringes are received by the two-divided detector, the variation of the wavelength can be detected.